1. Field of the Invention
The present invention relates to subterranean well cementing operations, and more particularly, to well cement compositions having improved corrosion resistance and methods of using such compositions in subterranean formations.
2. Description of the Prior Art
Hydraulic cement compositions are commonly utilized in subterranean operations, particularly subterranean well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby pipe strings such as casings and liners are cemented in well bores. In performing primary cementing, hydraulic cement compositions are pumped into the annular space between the walls of a well bore and the exterior surface of the pipe string disposed therein. The cement composition is permitted to set in the annular space, thereby forming an annular sheath of hardened substantially impermeable cement therein that substantially supports and positions the pipe string in the well bore and bonds the exterior surfaces of the pipe string to the walls of the well bore. Hydraulic cement compositions also are used in remedial cementing operations such as plugging highly permeable zones or fractures in well bores, plugging cracks in holes in pipe strings, and the like.
Portland cement is commonly used in subterranean cementing applications. Portland cements, however, may have limitations in certain applications, because they are prone to corrosive attacks by carbonic acid (H2CO3). Carbonic acid may be naturally present in a subterranean formation, or it may occur by the reaction of subterranean water and carbon dioxide (CO2), when the latter has been injected into the formation, e.g., as in a CO2-enhanced recovery operation. Carbonic acid is believed to react with the calcium hydroxide component of the Portland cement, which results in the cement becoming a soft amorphous gel. This is problematic because, inter alia, the permeability of the cement is increased. As a result, chloride and hydrogen sulfide ions, which may be present in the subterranean formation, may penetrate the cement sheath and adversely affect or react with the casing. The degradation of the cement can cause, inter alia, loss of support for the casing and undesirable interzonal communication of fluids.
The susceptibility of some cements, including Portland cements, to degradation by carbonic acid is especially problematic in some typical geothermal wells, as they involve very high temperatures, pressures and carbon dioxide concentrations. In such wells, cement failures often occur in less than five years, causing the collapse of the well casing. This in turn causes lost production and necessitates expensive casing repairs.